1. Field of the Invention
The present invention relates generally to check valves for restricting the direction of liquid flows in high pressure pumping systems, and more particularly to check valves for pumps used in liquid chromatography systems.
2. DESCRIPTION OF THE PRIOR ART
As depicted generally in FIG. 1a, liquid chromatography systems include reaction columns through which liquids are forced at high pressures, usually by pumps using reciprocating pistons and check valves to control flow direction. The pump driver 1 retracts a piston 2 from a cylinder chamber 3 to lower the cylinder pressure, thereby opening an inlet check valve 4 in an intake passage 5 through which liquid is drawn into the cylinder. Once the cylinder 3 is full, the pump driver advances the piston 2 to pressurize the cylinder to a high pressure, typically in excess of 1000 psi and possibly up to 10,000 psi, thereby closing the inlet check valve 4 and opening an outlet check valve 6 in an exhaust passage through which pressurized liquid is delivered to the column 8.
A check valve operates with a ball which temporarily mates against a seat to seal a passage through the underlying center of the seat. More specifically, as shown in FIG. 1 a conventional check valve 10 is comprised of a body 12 with a cylindrical bore 14 formed between an inlet port 16 and an outlet port 18. Disposed within bore 14, in ascending order, are a retainer 20, a spacer 22, lower check valve 24 parts including a lower seat 26, a lower ball 28 and a lower seat holder 30, a spacer 32, upper check valve 34 parts including an upper seat 36, an upper ball 38 and an upper seat holder 40, and a spacer 42. Upper check valve 34 is identical to lower check valve 24.
Referring to FIG. 2, the valve 10 is exploded to show its parts more clearly. When valve 10 is installed in a pump the body 12 is threadedly mated with a pump housing (not shown) which includes a shoulder serving to retain the parts within bore 14.
FIG. 3 is a partially broken away cross section detailing the interface between, for example, seat 26 and ball 28. Seat 26 has an inside wall 50 defining an axial passage 52. The upper end of wall 50 opens into a chamfered surface 54 which spreads outward to a planar end face 56. Chamfered surface 54 has a mid-portion 58 which is concave and complementary to the surface 60 of ball 28. The lower and upper margins 62 and 64 of surface 54 are rounded to form smooth transitions to wall 50 and to end face 56 respectively.
Useful measurements in liquid chromatography systems require stable solvent flow rates which depend in part upon the amount of leakage, or conversely, the integrity of seals in the check valves. Seal integrity is a function of the tolerance between the (ideally) complementary shapes of seating surfaces 58 and 60 on the seat and ball respectively. For the valve 10 to seal tightly, chamfered surface portion 58 must be well defined and precisely complementary to ball surface 60.
To operate with corrosive liquids check valves are made from inert materials. Conventional stainless steel and other metal alloy seats are not durable enough to maintain their shapes through repeated pounding usage, and tend to become pitted and wear irregularly. As the shape of the seat deteriorates high pressure liquids leak through gaps in the interface between sealing surfaces 58 and 60. Consequently, metal alloys have been widely supplanted by synthetic sapphire and/or ruby crystals which are more rigid and retain their shapes better for more consistently sealing balls and seats.
Sapphire and ruby crystals have varying hardness at various angles to their crystal growth axes, making them difficult to grind, although it is possible to grind sapphire and ruby crystals to tolerances required for check valve balls. However, check valve seats have complex chamfered profiles and have therefore been machined using a diamond broach which scores the chamfered surface 54 with microscopic grinding marks as shown in FIG. 4a.
Relevant prior art valves using sapphire or ruby crystal balls and/or seats are disclosed in the following patents. U.S. Pat. No. 4,094,337 describes a bleed valve using a ball and seat formed from sapphire or ruby crystal to withstand high pressures encountered in steam boilers.
U.S. Pat. No. 4,139,469 describes a standard check valve using sapphire or ruby balls and seats and having an inlet filter to screen contaminant particles from the liquid solvent which might otherwise lodge on and interfere with sealing between the ball and seat surfaces.
In U.S. Pat. No. 4,282,897, interface surface seals are maintained by seats made of a soft metal, such as gold, which yields and conforms to the shape of a sapphire ball surface if the seat surface picks up contaminant particles.
Although sapphire/ruby crystals are strong enough to maintain their shape and sealing ability, it is well known that mixtures of acetonitrile (MECN) and water in a high pressure liquid chromatography mobile phase (generally in a gradient of changing proportions) sometimes cause inlet check valves to stick shut. Heretofore, this problem has been dealt with by replacing stuck valves with new valves as often as once a week until finding a valve that does not stick.
There remains therefore a need for a liquid chromatography pump flow check valve which will not stick shut when operated with mixtures of acetonitrile and water.